Funny, isn't it? Why the double-standard? Some of this is just politics, sure. But mostly, it's just unconscious assumptions based on what we've grown up to expect is normal.
Men and women alike absorb these assumptions from childhood. Unless you really think about it, you probably won't even notice them. Why assume that grandchildren will affect Hillary Clinton's political career, when it had zero effect for Mitt Romney or any other man? That assumption is sexist, even if our nation is sexist enough that it will have an effect. (Our media are, after all, already making a big deal about it.)
Our very language perpetuates stereotypes. 'Pussy' and 'dick' are both negative connotations, but they mean very different things - and it's not a coincidence which is which. That's why I never describe courage as having 'balls.' Women have at least as much courage as men, although even women may think they shouldn't show it.
And speaking of courage, note that, if men got pregnant, there would be zero controversy about birth control. That would be absolutely inconceivable. But because it's just women who get pregnant, that's 'God's will,' right? It's their own fault, anyway, for having sex. (Even women often think that women - other women, at least - shouldn't be having sex - not without some kind of punishment, at least.)
Sexism is like racism - we've made great progress, but we've got a long way to go. To progress further, we must recognize the problem, so I applaud Jon Stewart here.